I reckon no one broke the agreements made at the Commission meeting
There was no agreement binding Vito either to make the Corleones top Mafia family again but he needed never wanted this for you son Michael to pull it off
The Dons respected each other's territories but would never pass opportunities to chisel each other's territories even in peace times more so when they smell blood in apparent weakness Barzini was always wanting what Vito had achieved, to be the Top Mafia Don